In either of the states you mentioned, could you post the output of
Thereby, you get
information on the configuration of your X screens. xrandr is also the tool to configure all aspects of a multi-monitor setup.
Together with the information at https://wiki.archlinux.org/index.php/Xrandr
, you should be able to figure out how to correctly setup your monitors.
If you prefer a graphical utility, you can use arandr from the Debian standard repository, which provides a limited subset of xrandr's options.
(Depending on the xrandr output) your issue is more likely a userland software configuration thing than something driver/kernel related.
I hope this helps.